1. Field of the Invention
This invention is in the general field of semiconductor devices and more particularly in the field of protection of semiconductor devices from thermal and current overloads.
2. Description of the Prior Art
Semiconductor devices in general, and power transistors in particular, are subject to high load currents which require the device to normally operate at or near their permissible junction temperature.
All too often, while operating at or near the permissible junction temperature, the device will be subjected to a rapid increase in load current which causes the junction temperature to rapidly increase above the permissible temperature resulting in thermal deterioration or destruction of the device.
As one method of solving the above-mentioned problem, there has been previously proposed the disposing of either a negative temperature characteristic (NTC) thermistor or a positive temperature characteristic (PTC) thermistor in the vicinity of a body of semiconductor material that is the main component of the transistor.
When the negative temperature characteristic thermistor is used, the base current through the transistor can be shunted to the emitter side thereof through the utilization of a change in its resistance due to a temperature rise. Alternatively where the positive temperature characteristic thermistor is used the PTC thermistor may be serially connected to the base of the transistor to suppress the base current through the transistor.
There has also been proposed a method wherein a thermally sensitive diode is connected between an emitter and a base of a transistor to be protected from overheating, so as to be put electrically in parallel to an emitter junction of the transistor. By utilizing a decrease in forward impedance of the diode due to a rise in temperature, the base current through the transistor is shunted to the emitter side of the transistor when the transistor has risen in temperature whereby an increase in collector current is suppressed.
In the prior art methods described above, a thermistor or a thermally sensitive diode is used as a thermally sensitive element. These thermally sensitive elements have their impedances slowly varied with their increased temperature and the change is usually insufficient to help when it is finally realized. Therefore in view of their performance, the prior art thermally sensitive elements, thermistors and diodes are not satisfactory in many cases in that, when a transistor has a temperature sufficiently less than its permissible junction temperature, current amplification factor is lowered, alternatively, even at excessively high temperatures, the collector current of the transistor to be protected cannot be sufficiently reduced and it is impossible to ensure in all cases that the temperature of the transistor will be prevented from rising beyond its permissible junction temperature.